From what I know about biology, I'm pretty sure that we are descendants of a unicellular organism that ate ancestors of mitochondria. Our ancestors got to use super high power generated by mitochondria's oxygen-base metabolism, so they could move better, hunt better, evade predators better, and in time mutated into us and all living things that have mitochondria in their cells. (By the way, all mitochondria in you come from your mother; your father's sperm was too small to carry any mitochondrion. When my kids misbehave, I would turn to my wife and tell her "Titus/Tatia/Tanya got your mitochondria.")
I just read an interesting account of something analogous to what our unicellular ancestors might have done: a carnivorous cell hunts other things until it eats a particular type of unicellular plants; it then turns into a photosynthesizing organism and stops hunting. When it reproduces (by binary fission), one offspring becomes a hunter, the other becomes a plant.
Two researchers have shown a striking example of endosymbiosis forming now: in 2005 Noriko Okamoto an Isao Inouye reported on a unicellular organism called Hatena. Hatena (”enigma” in Japanese) leads a curious life cycle. Hatena is a single-cell organism, swimming around in the water, using a little feeding apparatus to eat cells and organic material smaller than itself. At some point, it would feed on another unicellular algae, the Nephroselmis. Once Hatena swallows Nephroselmis, it does not digest it. Rather, Nephrosolmis makes itself comfortable home inside Hatena. The alga starts growing inside Hatena: it grows to about 10 times its original size, filling up most of Hatena. The alga also seems to lose most of its own organelles, except for the chloroplast. The chloroplast actually grows bigger.
Hatena changes too as a result. Before ingesting the alga, it has a rather complex “mouth”, or feeding apparatus. After ingesting the algae, this mouth disappears only to be replaced by an eyespot from the algae. The eyespot is a light sensing organelle, a very primitive eye, that guides algae to light sources. In this case, it also guides the host, Hatena, to light. Hatena has obviously stopped feeding, and least through its mouth. It is now swimming to the light, letting the alga photosynthesize its food for both of them.